Platinum-Group Elements and Minerals in the Lower and Middle Group Chromitites of the Bushveld Complex, South Africa

The 2,050 Ma old Bushveld Complex in South Africa is the largest layered mafic-ultramafic intrusion on Earth and contains the largest resources of platinum-group elements (PGE) as well as economically important reserves of chromium and vanadium. The correlation between chromite and PGE is remarkably well expressed as even the thinnest chromitite stringers in the Bushveld Complex contain elevated concentrations of PGE. The UG-2 chromitite has been investigated in great detail, however, studies on the distribution of PGE and the mineralogical character of the PGM in the LG and MG chromitites are scarce and the knowledge of the distribution and the mineralogical siting of the PGE in the chromitite seams of the Lower Group (LG) and Middle Group (MG) of the Bushveld Complex is limited.
In the present study, concentrates from the LG-6 and MG-2 chromitites of the Bushveld Complex are studied to determine the distribution of the PGE and the nature of the platinum-group mineral (PGM) in these chromitite seams. Quantitative mineralogical analysis using the Mineral Liberation Analyser (MLA) software was carried out in order to determine the PGM proportions in these concentrates. The dominant PGM are sulfides, namely cooperite-braggite, malanite-cuprorhodosite, and laurite, followed by PGE-sulfarsenides, sperrylite, and Pt-Fe alloys. The additional information of PGM area percentages compared to frequency data obtained by the MLA data, show that in LG and MG chromitites of the western Bushveld Complex laurite grains make up the largest area amount of PGM. The matching sets of PGM present in the LG and MG chromitites of both the western and the eastern Bushveld Complex, and the UG-2 chromitite show far-reaching similarities which support the indication of a characteristic, general chromitite-related PGM assemblage. These PGM in the LG and MHR are mostly associated with silicates (54 and 60% in the LG-6 and MG-2, respectively), followed by sulfides (32 and 37% in the MG-2 and LG-6, respectively). Rarer associations are with chromite (2%) or other PGM (5 to 7%). Palladium and Rh contents in sulfides (mainly pentlandite) may range up to thousands of ppm in ores associated with mafic-ultramafic intrusions. Palladium and Rh contents in pentlandite of the LG and MG chromitites of this study are low and erratic although maximum contents of 7731 ppm Pd and 6023 ppm Rh were detected. Rare thiospinels of the polydymite-linnaeite-greigite series achieve PGE contents of 1428 ppm Pt, 5370 ppm Rh and 1460 ppm Pd.